I think "Call Police" is the best choice. There aren't many people who would pass that sign and not call it in. You're not asking much of them, really. Clear, direct, and clean from the point of view of the passer-by.

"Help" is too vague (okay, it's not really, but we live in crazy times). I think many would interpret it as "stop and help me," which has a lot more risk/hassle/ick factors, including lawyers and axe murderers. Not the calculation you want going on in the heads of your potential rescuers.